How Stuff Works is provides “credible, unbiased, and easy-to-understand explanations of how the world actually works”. I often use short videos from this site to assist with the explanation of a concept to my students. While probably more suited for middle primary to upper high school students, I have found some good content to use with my Grade Two students.
“Howcast provides the answer to any how-to question” and encourages people to submit their own how-to videos on the site. My students have loved some of the procedural videos in the “kids” category on this site such as the origami, social skills, cooking, craft and science videos.
Both of these sites need to be monitored by the teacher for inappropriate content (particularly Howcast, from my experience). I prefer to chose the videos I want my students to watch and show them on the IWB, iPod Touch or classroom computer.
Here is an example of a Howcast video that I showed as a introduction to a lesson on length. My students had to make a paper plane and measure and record how far they could fly their plane.